The present invention relates to the extraction of heavy nonferrous metals from their ores and, in particular, to a means of absorbing excess heat during the continuous, autogenous conversion of the sulfides of these metals.
In the process for continuous, autogenous conversion of nonferrous metal sulfides to recover the value metals therefrom, known as, and referred to hereinafter as, the "Q-S Oxygen Process," oxygen and a shielding fluid, heretofore generally being sulfur dioxide, are introduced into an elongated, rounded cross-section furnace at various locations. This process, especially useful in the conversion of copper sulfide concentrates to crude copper, combines much of the latest pyrometallurgical processing technology into a continuous copper making process. This process is generally described in U.S. Pat. No. 3,941,587 and U.S. Pat. No. 3,988,148, the contents of both patents being incorporated herein by reference.
One of the features of the Q-S Oxygen Process is the use of fluid shielded nozzles for injecting oxygen-containing gases into the molten charge. The use of a shielding fluid keeps the temperature of these nozzles at a permissible level so that sufficiently high concentrations of oxygen may be employed, and so that the process is autogenous, that is, not requiring external fuels for purposes of heat. The aforementioned patents also disclose that in addition to sulfur dioxide, hydrocarbons or water may be used as a shielding fluid.
It is found, furthermore, that in addition to merely shielding the nozzles of the oxygen injectors, it is necessary to remove heat from the entire system in general. That is, the iron and sulfur content of the typical ore concentrates which are produced by present day flotation processes and which are used as feed material in the Q-S Oxygen Process are sufficiently high so that the oxidation of these elements with the pure oxygen employed in the Q-S Oxygen Process will result in the generation of more heat than is consumed by the process. The result of the production of such excess heat is that the molten charge temperature rises to a level which may adversely affect the furnace refractories if heat were not removed from the system.
Several methods are known for the absorption and removal of this excess heat. For example, injection of a heat absorbing material such as nitrogen, water, copper scrap or recycled sulfur dioxide may be effected. These methods, however, have certain disadvantages. The use of nitrogen or water will tend to dilute the concentration of sulfur dioxide in the system's exhaust gas, thus making the recovery of some useful product from that sulfur dioxide more expensive. Specifically, if the exhausted sulfur dioxide is to be used to produce sulfuric acid, water vapor must be removed. If the exhausted sulfur dioxide is to be used to produce elemental sulfur, the presence of nitrogen would require the use of considerably more energy than if the same process were carried out on a sulfur dioxide stream containing little nitrogen. While the addition of copper scrap to the process for absorption of heat is a preferred method in that the off-gases are not diluted, such scrap is rarely available in sufficient quantity and in the physical condition that is required at a converter site.
The recycling of sulfur dioxide to the system also prevents dilution of the exhaust gas. The comparatively limited heat absorption capacity of sulfur dioxide, however, requires that it be injected into the reactor in such quantities that the volume of the exhaust gas, which includes the sulfur dioxide injected as a protective fluid as well as that generated in the process itself, may be disadvantageously large. That is, the subsequent removal of dust from the exhaust gas by means of a precipitator or equivalent means may be more efficiently accomplished where a given amount of dust is carried by a comparatively small rather than a large volume of exhaust gas. In the same way, the subsequent recovery of waste heat may also be facilitated where the volume of the exhaust gas is limited.
It is, then, an object of the present invention to provide a means whereby the nozzles employed for injecting oxygen directly into the molten charge in a Q-S Oxygen Process are shielded and in which the excess heat generated by that process is efficiently removed from the reactor, while at the same time limiting the volume of the exhaust gas which is produced by the process. A further object of the present invention is to provide a means for achieving such cooling in which the process exhaust gas is equally well suited for the production of either elemental sulfur or sulfuric acid. Still further objects and advantages of the present invention will be apparent from the following description.